It's really hard, op. I became chronically ill about the same time as you (like you, fit and active beforehand) and it's really changed my friendships. Admittedly, friends were great for a long time but a couple of my closest no longer seem to be interested if I can't join them for karaoke etc.
I don't know whether your friends are meaning to pressure you because they don't understand and assume failure to attend will be a failure of friendship on your part, or because they want you to be included and are conscious of you not being...
All this to say, I'm guessing they probably don't realise what day to day life is like for you, only seeing you on good days and not witnessing the consequences of pushing yourself. Have you explained?
It's easy to assume friends pick it up from comments we make but I would guess you don't go on about it much and try to live in the moment. It's also hard to open up and be vulnerable that way.
Weddings do strange things to people and lots of women seem to become massively entitled. This could also be a flash point in your friendship where you see your friendship is more fairweather than you thought.
Put yourself first, do your best to explain and, perhaps, if you can't make it and the bride doesn't disgrace herself with her reaction, send flowers in your place.
While we should never have to apologise for disabilities, sometimes it doesn't hurt to smooth the waters- especially at a time where some people lose their minds over trivial plans.